Glass substrate for information recording medium and information recording medium

ABSTRACT

A glass substrate for information recording medium, said glass substrate being composed of an aluminosilicate glass containing 60-75% by mass of SiO 2 , 5-18% by mass of Al 2 O 3 , 3-10% by mass of Li 2 O, 3-15% by mass of Na 2 O and 0.5-8% by mass of ZrO 2  relative to the entire glass components. The glass substrate for information recording medium contains neither As (arsenic) nor Sb (antimony), while containing at least one substance selected from the group consisting of SO 3  (sulfurous acid), F (fluorine), Cl (chlorine), Br (bromine) and I (iodine), as a refining agent. The molar ratio of the total amount of the refining agent to the amount of Al 2 O3 is within the range of 0.02-0.20.

This application is a National Stage of International ApplicationPCT/JP2009/062188 filed with Japanese Patent Office on Jul. 3, 2009.

TECHNICAL FIELD

The present invention relates to a glass substrate for an informationrecording medium such as a magnetic disk and an information recordingmedium using the same, and more specifically to a glass substrate for aninformation recording medium made of aluminosilicate glass and aninformation recording medium using the same.

BACKGROUND

Of information recording media having a recording layer utilizingmagnetic, optical, or magneto-optical properties, magnetic disks areavailable as typical media. Conventionally, as magnetic disk substrates,aluminum substrates have been widely used. However, over recent years,with the demand for the reduction of magnetic head floating amount forrecording density enhancement, glass substrates have been increasinglyused, which glass substrates exhibit superior surface flatness toaluminum substrates and have less surface defects. Of these, preferablyused are glass substrates formed of aluminosilicate glass capable ofstrengthening substrates by chemical strengthening treatment by ionexchange, because of their enhanced impact resistance and vibrationresistance.

In such a glass substrate for an information recording medium, to becapable of high density recording with reduced surface defects, it isnecessary that gas bubbles generated in the melting process of glass areallowed to be present in a glass substrate at as lowest level aspossible. Conventionally, a method has been commonly employed in whichAs₂O₃ and Sb₂O₃ are contained in a glass component as fining agents,whereby gas bubbles in molten glass are removed (clarified) (forexample, refer to Patent Document 1).

However, since As₂O₃ and Sb₂O₃ are toxic, from the environmental andhealth viewpoints, a tendency to regulate the usage thereof is beingwidespread. Therefore, studies have been made to realize a method toremove gas bubbles in the molten glass without using As₂O₃ or Sb₂O₃serving as fining agents is studied (for example, Patent Document 2),and proposed is a method to remove gas bubbles by depressurizing moltenglass.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: Unexamined Japanese Patent Application    Publication No. 8-321034-   Patent Document 2: Patent Document 1: Unexamined Japanese Patent    Application Publication No. 2000-128549

BRIEF DESCRIPTION OF THE INVENTION Problems to be Solved by theInvention

However, according to the method described in Patent Document 2, therehave been such problems that a complex process and a specialdecompression degassing apparatus are required and also a glasscomponent tends to be changed by volatilization of the glass componentwith pressure reduction.

In view of the above technological problems, the present invention wasconceived. An object of the present invention is to provide a glasssubstrate for an information recording medium with sufficiently removedgas bubbles in which neither As (arsenic) nor Sb (antimony) element iscontained; and an information recording medium using the same.

Means to Solve the Problems

To solve the above problems, the present invention has the followingfeatures.

Item 1. A glass substrate for an information recording medium made ofaluminosilicate glass containing 60% to 75% by mass of SiO₂, 5% to 18%by mass of Al₂O₃, 3% to 10% by mass of Li₂O, 3% to 15% by mass of Na₂O,and 0.5% to 8% by mass of ZrO₂ with respect to whole glass components,comprising:

neither As (arsenic) element nor Sb (antimony) element;

at least one type selected from the group consisting of SO₃ (sulfurousacid), F (fluorine), Cl (chlorine), Br (bromine), and I (iodine), asfining agent,

wherein a molar ratio of a whole amount of the fining agent to theA₂lO₃, which is (the whole amount of the fining agent)/A₂lO₃, is withina range of 0.02 to 0.20.

Item 2. The glass substrate for an information recording medium of item1, wherein the fining agent contains SO₃ (sulfurous acid).

Item 3. The glass substrate for an information recording medium of item1 or 2, wherein a total content of the fining agent is in a range of 1%by mass or less with respect to the whole glass components.

Item 4. The glass substrate for an information recording medium of anyone of items 1 to 3, wherein the glass substrate for an informationrecording medium is chemically strengthened by ion exchange.

Item 5. An information recording medium, comprising:

a recording layer provided on the glass substrate for an informationrecording medium of any one of items 1 to 4.

Advantages of the Invention

According to the present invention, since predetermined polyvalentelements made of oxides functioning as fining agents in glass arecontained at a molar ratio of a predetermined range to Al₂O₃ in theglass components, clarification reaction due to the valence change ofthe polyvalent elements can effectively function. Therefore, without As(arsenic) or Sb (antimony) element, a glass substrate for an informationrecording medium with sufficiently removed gas bubbles can be obtained.

PREFERRED EMBODIMENTS OF THE INVENTION

Preferred embodiments of the present invention will now be described indetail.

(Glass Substrate for an Information Recording Medium)

The glass substrate for an information recording medium of the presentinvention is made of an aluminosilicate glass containing 60-75% by massof SiO₂, 5-18% by mass of Al₂O₃, 3-10% by mass of Li₂O, 3-15% by mass ofNa₂O, and 0.5-8% by mass of ZrO₂ with respect to the total glasscomponents. Therefore, chemical strengthening treatment by ion exchangecan be applied thereto, whereby enhanced impact resistance and vibrationresistance can be ensured. The reason why each component was regulatedin the above range is as follows:

SiO₂ is a critical component to form a network structure of glass andsignificantly contributes to chemical resistance. When the content ofSiO₂ is less than 60% by mass, chemical resistance may be lower. Incontrast, in the case of more than 75% by mass, melting temperature isexcessively high. Therefore, the content of SiO₂ needs to fall in arange of 60-75% by mass. In this range, a range of 60-71% by mass ispreferable.

Al₂O₃ is a critical component to form a network structure together withSiO₂, functioning to enhance not only chemical resistance but also ionexchange performance. When the content of Al₂O₃ is less than 5% by mass,chemical resistance and ion exchange performance may be lower. Incontrast, in the case of more than 18% by mass, devitrificationresistance is lower. Therefore, the content of Al₂O₃ needs to fall in arange of 5-18% by mass. In this range, a range of 9-14% by mass ispreferable.

LiO₂ is a necessary component to carry out chemical strengtheningtreatment by ion exchange. In the chemical strengthening treatment, Li⁺ions in glass are ion-exchanged with Na⁺ ions or K⁺ ions contained in achemical strengthening treatment liquid, whereby a glass substrate isstrengthened. When the content of LiO₂ is less than 3% by mass, this ionexchange performance is lower. In contrast, in the case of more than 10%by mass, devitrification resistance and chemical resistance are lower.Therefore, the content of LiO₂ needs to be 3-10% by mass. In this range,a range of 4-6% by mass is preferable.

Na₂O is a necessary component to carry out chemical strengtheningtreatment by ion exchange. In the chemical strengthening treatment, Na⁺ions in glass are ion-exchanged with K⁺ ions contained in a chemicalstrengthening treatment liquid, whereby a glass substrate isstrengthened. When the content of Na₂O is less than 3% by mass, this ionexchange performance is decreased and also devitrification resistance islower. In contrast, in the case of more than 15% by mass, chemicalresistance is decreased. Therefore, the content of Na₂O needs to be3-15% by mass. In this range, a range of 6-10% by mass is preferable.

ZrO₂ is a necessary component to enhance chemical resistance. When thecontent of ZrO₂ is less than 0.5% by mass, chemical resistance is lower.In contrast, in the case of more than 8% by mass, devitrificationresistance is lower. Therefore, the content of ZrO₂ needs to be 0.5-8%by mass. In this range, a range of 1-7% by mass is preferable.

Either As₂O₃ or Sb₂O₃ is not contained. Herein, “being not contained”refers to exclude intentional addition of those elements as rawmaterials of glass. Allowable is a trace amount contained to the extentthat they are inevitably contained as impurities in raw materials ofother components.

The investigation results obtained by the present inventors made itclear that in a glass substrate containing each of the above components,predetermined fining agent was contained at a molar ratio in apredetermined range to Al₂O₃ in the glass components, whereby without As(arsenic) or Sb (antimony) element, gas bubbles in the glass weresufficiently removed. Namely, the glass substrate for an informationrecording medium of the present invention contains as fining agent atleast one selected from the group consisting of SO₃ (sulfurous acid), F(fluorine), Cl (chlorine), Br (bromine), and I (iodine). The molar ratioof the total amount of these fining agents to the amount of Al₂O₃ in theglass components, which is (the total amount of the finingagents)/Al₂O₃, falls in a range of 0.02-0.20.

The reason why gas bubbles can be sufficiently removed as describedabove when predetermined fining agents are contained at a molar ratio ina predetermined range to Al₂O₃ in the glass components is basicallyconsidered as follows.

In general, a fining agent contributes to removing (clarifying) gasbubbles present in molten glass by the following two functions:

(a) The first one of the functions is a function to generate gas inmolten glass in the process of raising the temperature of the moltenglass. Gas bubbles in the molten glass move upward by the buoyancythereof to reach the surface of the molten glass, resulting in burstingand disappearance. Herein, the ascending velocity of the gas bubbles inmolten glass largely depend on the size of the gas bubbles. Large gasbubbles relatively easily reach the surface due to large ascendingvelocity thereof. However, small gas bubbles have small ascendingvelocity, whereby an extremely long time is required to reach thesurface. In the process of raising the temperature of molten glass, gasis generated from a fining agent, whereby gas bubbles in the moltenglass grow larger and then the ascending velocity of the gas bubbles isincreased, resulting in accelerating the disappearance of the gasbubbles.

Any of SO₃ (sulfurous acid), F (fluorine), Cl (chlorine), Br (bromine),and I (iodine), which are used in the present invention, has a featurethat the solubility is lower at the higher the temperature of the moltenglass, and gas is generated in the course of raising the temperature ofthe molten glass.

(b) The second one of the functions is a function to absorb gas presentin molten glass in the process of decreasing the temperature of themolten glass. Any of the aforementioned fining agents has a feature thatthe solubility is higher at the lower temperature of the molten glass,and the bubbles are absorbed in the molten glass and disappear in thecourse of decreasing the temperature of the molten glass.

In order to sufficiently remove gas bubbles in molten glass as describedabove, it is necessary that the generation and absorption of gas by thechange in the solubility of the fining agent are effectively carriedout. The present inventors conducted diligent investigations, withrespect to the aforementioned fining agents, on the phenomenon of thechange of the solubility into the molten glass depending on thetemperature, and found that the change in solubility was largelyaffected by the redox reaction of other metal ions, specifically Al ionscoexisting in molten glass. Then, investigations were further conducted,whereby it was found that in an aluminosilicate glass havingpredetermined components, when the molar ratio of the total amount ofthe above fining agents to the amount of Al₂O₃, which is (the totalamount of the fining agents)/Al₂O₃, fell in a range of 0.02-0.20,clarification reaction by the change in solubility was effectivelyperformed.

As the fining agents, at least one type selected from the groupconsisting of SO₃ (sulfurous acid), F (fluorine), Cl (chlorine), Br(bromine), and I (iodine). Only one type of polyvalent element may beused alone, or two or more types may be used together. Of these, SO₃(sulfurous acid) is specifically preferable since it can effectivelyremove gas bubbles.

When the molar ratio of the total amount of the fining agents to theamount of Al₂O₃, which is (the total amount of the fining agents)/Al₂O₃,falls outside a predetermined range, clarification reaction by thevalence change is inadequate, whereby it becomes difficult tosufficiently remove gas bubbles in molten glass. Accordingly, the molarratio to Al₂O₃ needs to fall in a range of 0.02-0.50, more preferably ina range of 0.05-0.15.

In addition, it is more preferable that the total content of theaforementioned fining agents is at most 1% by mass with respect to thetotal glass components. When the content of the fining agents is limitedto at most 1% by mass, the fining effect sufficiently works, withoutdecreasing devitrification resistance.

Further, the shape of a glass substrate is not specifically limited.However, a disk-shaped substrate having a central hole is common. Thesize and thickness of a glass substrate are not specifically limited.For example, the outer diameter is 2.5 inches, 1.8 inches, 1 inch, or0.8 inch and the thickness is 1 mm, 0.64 mm, or 0.4 mm.

(Production Method of a Glass Substrate for an Information RecordingMedium)

As described above, in the glass substrate for an information recordingmedium of the present invention, gas bubbles can be sufficiently removedby the function of predetermined fining agents. Thereby, neither acomplex process nor a special apparatus for production is required,whereby production can be carried out using a well-known, commonproduction method.

Usually, it is common that a blank material as the base of a glasssubstrate for an information recording medium is prepared andthereafter, production is carried out in processes such as inner andouter circumference processing, grinding and polishing processing,chemical strengthening treatment, and cleaning. With regard to thepreparation of the blank material, a method of preparation by pressmolding of molten glass and a method of preparation by cutting sheetglass are known. The inner and outer circumference processing is aprocess to carry out boring processing of a central hole, grindingprocessing to form the shape of the inner and outer circumferences andto ensure dimensional accuracy, and polishing process of the inner andouter circumferences. The grinding and polishing processing is a processto carry out grinding processing and polishing processing to satisfy theflatness and the surface roughness of a surface on which a recordinglayer is formed. Usually, the process is frequently carried out bydividing into some stages such as coarse grinding processing, finegrinding processing, primary polishing processing, and secondarypolishing processing. The chemical strengthening treatment is a processto immerse a glass substrate in a chemical strengthening treatmentliquid to strengthen the same. And, the cleaning is a process to removeabrasives remaining on the surface of a glass substrate and foreignsubstances such as a chemical strengthening treatment liquid.

In particular, since the glass substrate for an information recordingmedium of the present invention is made of aluminosilicate glasscontaining predetermined glass components, enhanced impact resistanceand vibration resistance can be ensured by chemical strengtheningtreatment. The chemical strengthening treatment is carried out by an ionexchange method in which a glass substrate is immersed in a heatedchemical strengthening treatment liquid, whereby lithium ions and sodiumions being components of the glass substrate are exchanged with ionssuch as sodium ions and potassium ions having larger ion radiuses thanthese ions. Strain produced by the ion radius difference generatescompressive stress in the region where the ions have been exchanged,whereby the surface of the glass substrate is strengthened.

As the chemical strengthening treatment liquid, a molten salt containingsodium ions or potassium ions is commonly used. For example, a nitrate,a carbonate, and a sulfate of sodium or potassium, and a mixed meltedsalt thereof are cited. Of these, from the viewpoint of low meltingpoint and being able to prevent the deformation of a glass substrate, amixed molten salt of sodium nitrate (NaNO₃) and potassium nitrate (KNO₃)is preferably used.

(Information Recording Medium)

An information recording medium can be produced by forming at least arecording layer on the above glass substrate for an informationrecording medium. The recording medium is not specifically limited, andvarious types of recording layers utilizing magnetic, optical, ormagneto-optical properties are usable. An information recording medium(a magnetic disk), employing a magnetic layer as a recording layer, isespecially suitable for production.

Magnetic materials used for such a magnetic layer are not specificallylimited, and well-known materials can be appropriately selected andused. However, to realize enhanced retention power, a Co-based alloycontaining Co exhibiting large crystal anisotropy as the main componentis suitable. In particular, listed are CoPt, CoCr, CoNi, CoNiCr, CoCrTa,CoPtCr, CoNiPt, CoNiCrPt, CoNiCrTa, CoCrPtTa, and CoCrPtSiO. Further,employable is a multi-layer structure (for example, CoPtCr/CrMo/CoPtCror CoCrPtTa/CrMo/CoCrPtTa) to reduce noise by dividing a magnetic layerby a non-magnetic material (for example, Cr, CrMo, or CrV).

As the magnetic layer, other than the above Co-based materials,ferrite-based or iron-rare-earth-based materials, or granular materialshaving a structure in which magnetic particles of Fe, Co, CoFe, orCoNiPt are dispersed in a non-magnetic film formed of SiO₂ or BN arealso usable. The magnetic layer may employ either one of an in-planetype and a vertical type recording format.

As a formation method for a magnetic layer, any appropriate well-knownmethod is usable. For example, cited are a method to spin-coat with athermally curable resin, in which magnetic particles are dispersed, on aglass substrate for an information recording medium, a sputteringmethod, and an electroless plating method. From the viewpoint of thinnerfilm and higher density realization of a magnetic layer, the sputteringmethod or the electroless plating method is preferable.

For a magnetic disk, an underlayer is further provided as appropriate.As materials for the underlayer, non-magnetic metals such as Cr, Mo, Ta,Ti, W, V, B, Al, and Ni are listed. When a magnetic layer contains Co asthe main component, from the viewpoint of magnetic characteristicenhancement, a material for the underlayer is preferably single Cr or Cralloy. Further, the underlayer is not limited to a monolayer and mayhave a multi-layer structure in which layers formed of the same ordifferent materials are layered. For example, Cr/Cr, Cr/CrMo, Cr/CrV,NiAl/Cr, NiAl/CrMo, and NiAl/CrV are cited.

Further, the surface of a magnetic layer may be provided with aprotective layer to prevent abrasion and corrosion of the magneticlayer. As the protective layer, for example, a Cr layer, a Cr alloylayer, a carbon layer, a hydrogenated carbon layer, a ZrO₂ layer, and anSiO₂ layer are cited. The protective layer may be a monolayer or have amultilayer structure containing layers of the same kind or differentkinds. Such a protective layer can continuously be formed using aninline-type sputtering apparatus, together with the underlayer and themagnetic layer. Further, when forming a protective layer of SiO₂ layeredon a Cr layer is formed, the SiO₂ layer may be formed in such a mannerthat a solution of tetraalkoxysilane diluted with an alcohol-basedsolvent and therein in which colloidal silica fine particles aredispersed, is coated on a Cr layer and then baked.

Still further, the surfaces of the magnetic layer and the protectivelayer may be provided with a lubricating layer to improve slippingproperties of a magnetic head. As the lubricating layer, examplesinclude a layer of a liquid lubricant containing perfluoropolyether(PFPE) applied and then subjected to a thermal treatment as appropriate.

EXAMPLES

Examples carried out to confirm the advantages of the present inventionwill now be described without limiting the present invention thereto.

Raw materials were blended according to the glass components shown inTable 1-Table 6 described below. As fining agents, SO₃ (Table 1), F(Table 2), Cl (Table 3), Br (Table 4), I (Table 5), and SO₃+F (Table 6)each were used. Herein, neither As nor Sb was not contained in anyfining agent above.

TABLE 1 The Number SiO₂ Al₂O₃ Li₂O Na₂O ZrO₂ Fining Agent of GasDevitrifi- (% by (% by (% by (% by (% by (% by Bubbles cation mass)mass) mass) mass) mass) Type mass) (molar ratio) (number) Properties No.1-1 Comparative 70.9 10.0 5.0 7.0 7.0 SO₃ 0.08 (SO₃/Al₂O₃) = 0.01 44 AExample No. 1-2 Example 70.9 10.0 5.0 7.0 7.0 SO₃ 0.16 (SO₃/Al₂O₃) =0.02 0 A No. 1-3 Example 70.4 9.9 5.0 6.9 6.9 SO₃ 0.78 (SO₃/Al₂O₃) =0.10 0 B No. 1-4 Example 69.9 9.8 4.9 6.9 6.9 SO₃ 1.55 (SO₃/Al₂O₃) =0.20 0 B No. 1-5 Comparative 69.6 9.8 4.9 6.9 6.9 SO₃ 1.93 (SO₃/Al₂O₃) =0.25 41 B Example No. 1-6 Comparative 62.9 14.0 6.0 10.0 7.0 SO₃ 0.11(SO₃/Al₂O₃) = 0.01 43 A Example No. 1-7 Example 62.9 14.0 6.0 10.0 7.0SO₃ 0.22 (SO₃/Al₂O₃) = 0.02 0 A No. 1-8 Example 62.3 13.8 5.9 9.9 6.9SO₃ 1.09 (SO₃/Al₂O₃) = 0.10 0 B No. 1-9 Example 61.6 13.7 5.9 9.8 6.8SO₃ 2.15 (SO₃/Al₂O₃) = 0.20 0 B No. 1-10 Comparative 61.3 13.6 5.8 9.76.8 SO₃ 2.68 (SO₃/Al₂O₃) = 0.25 36 B Example

TABLE 2 The Number SiO₂ Al₂O₃ Li₂O Na₂O ZrO₂ Fining Agent of GasDevitrifi- (% by (% by (% by (% by (% by (% by Bubbles cation mass)mass) mass) mass) mass) Type mass) (molar ratio) (number) Properties No.2-1 Comparative 71.0 10.0 5.0 7.0 7.0 F 0.02 (F/Al₂O₃) = 0.01 35 AExample No. 2-2 Example 71.0 10.0 5.0 7.0 7.0 F 0.04 (F/Al₂O₃) = 0.02 2A No. 2-3 Example 70.9 10.0 5.0 7.0 7.0 F 0.19 (F/Al₂O₃) = 0.10 1 B No.2-4 Example 70.7 10.0 5.0 7.0 7.0 F 0.37 (F/Al₂O₃) = 0.20 2 B No. 2-5Comparative 70.7 10.0 5.0 7.0 7.0 F 0.46 (F/Al₂O₃) = 0.25 39 B ExampleNo. 2-6 Comparative 63.0 14.0 6.0 10.0 7.0 F 0.03 (F/Al₂O₃) = 0.01 44 AExample No. 2-7 Example 63.0 14.0 6.0 10.0 7.0 F 0.05 (F/Al₂O₃) = 0.02 2A No. 2-8 Example 62.8 14.0 6.0 10.0 7.0 F 0.26 (F/Al₂O₃) = 0.10 1 B No.2-9 Example 62.7 13.9 6.0 9.9 7.0 F 0.52 (F/Al₂O₃) = 0.20 1 B No. 2-10Comparative 62.6 13.9 6.0 9.9 7.0 F 0.65 (F/Al₂O₃) = 0.25 46 B Example

TABLE 3 The Number SiO₂ Al₂O₃ Li₂O Na₂O ZrO₂ Fining Agent of GasDevitrifi- (% by (% by (% by (% by (% by (% by Bubbles cation mass)mass) mass) mass) mass) Type mass) (molar ratio) (number) Properties No.3-1 Comparative 71.0 10.0 5.0 7.0 7.0 Cl 0.03 (Cl/Al₂O₃) = 0.01 37 AExample No. 3-2 Example 71.0 10.0 5.0 7.0 7.0 Cl 0.07 (Cl/Al₂O₃) = 0.023 A No. 3-3 Example 70.8 10.0 5.0 7.0 7.0 Cl 0.35 (Cl/Al₂O₃) = 0.10 2 BNo. 3-4 Example 70.5 9.9 5.0 7.0 7.0 Cl 0.69 (Cl/Al₂O₃) = 0.20 3 B No.3-5 Comparative 70.4 9.9 5.0 6.9 6.9 Cl 0.86 (Cl/Al₂O₃) = 0.25 31 BExample No. 3-6 Comparative 63.0 14.0 6.0 10.0 7.0 Cl 0.05 (Cl/Al₂O₃) =0.01 42 A Example No. 3-7 Example 62.9 14.0 6.0 10.0 7.0 Cl 0.10(Cl/Al₂O₃) = 0.02 3 A No. 3-8 Example 62.7 13.9 6.0 10.0 7.0 Cl 0.48(Cl/Al₂O₃) = 0.10 2 B No. 3-9 Example 62.4 13.9 5.9 9.9 6.9 Cl 0.97(Cl/Al₂O₃) = 0.20 4 B No. 3-10 Comparative 62.2 13.8 5.9 9.9 6.9 Cl 1.20(Cl/Al₂O₃) = 0.25 36 B Example

TABLE 4 The Number SiO₂ Al₂O₃ Li₂O Na₂O ZrO₂ Fining Agent of GasDevitrifi- (% by (% by (% by (% by (% by (% by Bubbles cation mass)mass) mass) mass) mass) Type mass) (molar ratio) (number) Properties No.4-1 Comparative 70.9 10.0 5.0 7.0 7.0 Br 0.08 (Br/Al₂O₃) = 0.01 30 AExample No. 4-2 Example 70.9 10.0 5.0 7.0 7.0 Br 0.16 (Br/Al₂O₃) = 0.023 A No. 4-3 Example 70.4 9.9 5.0 6.9 6.9 Br 0.78 (Br/Al₂O₃) = 0.10 2 BNo. 4-4 Example 69.9 9.8 4.9 6.9 6.9 Br 1.54 (Br/Al₂O₃) = 0.20 2 B No.4-5 Comparative 69.6 9.8 4.9 6.9 6.9 Br 1.92 (Br/Al₂O₃) = 0.25 27 BExample No. 4-6 Comparative 62.9 14.0 6.0 10.0 7.0 Br 0.11 (Br/Al₂O₃) =0.01 35 A Example No. 4-7 Example 62.9 14.0 6.0 10.0 7.0 Br 0.22(Br/Al₂O₃) = 0.02 4 A No. 4-8 Example 62.3 13.8 5.9 9.9 6.9 Br 1.08(Br/Al₂O₃) = 0.10 2 B No. 4-9 Example 61.6 13.7 5.9 9.8 6.8 Br 2.15(Br/Al₂O₃) = 0.20 2 B No. 4-10 Comparative 61.3 13.6 5.8 9.7 6.8 Br 2.67(Br/Al₂O₃) = 0.25 37 B Example

TABLE 5 The Number SiO₂ Al₂O₃ Li₂O Na₂O ZrO₂ Fining Agent of GasDevitrifi- (% by (% by (% by (% by (% by (% by Bubbles cation mass)mass) mass) mass) mass) Type mass) (molar ratio) (number) Properties No.5-1 Comparative 70.9 10.0 5.0 7.0 7.0 I 0.12 (I/Al₂O₃) = 0.01 41 AExample No. 5-2 Example 70.8 10.0 5.0 7.0 7.0 I 0.25 (I/Al₂O₃) = 0.02 3A No. 5-3 Example 70.1 9.9 4.9 6.9 6.9 I 1.23 (I/Al₂O₃) = 0.10 2 B No.5-4 Example 69.3 9.8 4.9 6.8 6.8 I 2.43 (I/Al₂O₃) = 0.20 2 B No. 5-5Comparative 68.9 9.7 4.8 6.8 6.8 I 3.02 (I/Al₂O₃) = 0.25 39 B ExampleNo. 5-6 Comparative 62.9 14.0 6.0 10.0 7.0 I 0.17 (I/Al₂O₃) = 0.01 43 AExample No. 5-7 Example 62.8 14.0 6.0 10.0 7.0 I 0.35 (I/Al₂O₃) = 0.02 3A No. 5-8 Example 61.9 13.8 5.9 9.8 6.9 I 1.71 (I/Al₂O₃) = 0.10 3 B No.5-9 Example 60.9 13.5 5.8 9.7 6.8 I 3.37 (I/Al₂O₃) = 0.20 2 B No. 5-10Comparative 60.4 13.4 5.7 9.6 6.7 I 4.17 (I/Al₂O₃) = 0.25 37 B Example

TABLE 6 The Number SiO₂ Al₂O₃ Li₂O Na₂O ZrO₂ Fining Agent of GasDevitrifi- (% by (% by (% by (% by (% by (% by Bubbles cation mass)mass) mass) mass) mass) Type mass) (molar ratio) (number) Properties No.6-1 Comparative 71.0 10.0 5.0 7.0 7.0 SO₃ F 0.04 0.01 ((SO₃ + F)/Al₂O₃)= 0.01 42 A Example No. 6-2 Example 70.9 10.0 5.0 7.0 7.0 SO₃ F 0.080.02 ((SO₃ + F)/Al₂O₃) = 0.02 0 A No. 6-3 Example 70.7 10.0 5.0 7.0 7.0SO₃ F 0.39 0.09 ((SO₃ + F)/Al₂O₃) = 0.10 0 B No. 6-4 Example 70.3 9.95.0 6.9 6.9 SO₃ F 0.78 0.18 ((SO₃ + F)/Al₂O₃) = 0.20 0 B No. 6-5Comparative 70.1 9.9 4.9 6.9 6.9 SO₃ F 0.97 0.23 ((SO₃ + F)/Al₂O₃) =0.25 36 B Example No. 6-6 Comparative 63.0 14.0 6.0 10.0 7.0 SO₃ F 0.050.01 ((SO₃ + F)/Al₂O₃) = 0.01 43 A Example No. 6-7 Example 62.9 14.0 6.010.0 7.0 SO₃ F 0.11 0.03 ((SO₃ + F)/Al₂O₃) = 0.02 0 A No. 6-8 Example62.6 13.9 6.0 9.9 7.0 SO₃ F 0.55 0.13 ((SO₃ + F)/Al₂O₃) = 0.10 0 B No.6-9 Example 62.2 13.8 5.9 9.9 6.9 SO₃ F 1.08 0.26 ((SO₃ + F)/Al₂O₃) =0.20 0 B No. 6-10 Comparative 61.9 13.8 5.9 9.8 6.9 SO₃ F 1.35 0.32((SO₃ + F)/Al₂O₃) = 0.25 37 B Example

Each raw material was put into a melting furnace heated at 900° C.-1300°C., followed by melting, clarification, and homogenization by stirring,and then the resulting molten glass was press-molded to prepare a blankmaterial. Thereafter, by inner and outer circumference processing, aswell as grinding and polishing processing, a glass substrate having anouter diameter of 65 mm, an inner diameter of 20 mm, and a thickness of0.635 mm was produced.

With regard to each of the thus-produced glass substrates, the number ofresidual gas bubbles and devitrification properties during melting wereevaluated. The evaluation of the number of residual gas bubbles wasconducted in such a manner that the number of gas bubbles per glasssubstrate was counted with respect to the entire surface of a glasssubstrate by using an optical microscope of a magnification of 50. Theevaluation of the devitrification properties was conducted by judgingthe numerical value of the difference ΔT (° C.) between T_(log η=2.5) (°C.) and T_(L) (° C.) (ΔT=T_(log η=2.5)−T_(L)), where a temperature atwhich the melting viscosity of a glass satisfies the expression of logη=2.5 is T_(log η=2.5) (° C.) and the liquid-phase temperature is T_(L)(° C.). When the relationship 150° C.≦ΔT was satisfied, the evaluationresult was the best (A). When the relationship 50° C.≦ΔT<150° C. wassatisfied, the evaluation result was good (B). When the relationshipΔT<50° C. was satisfied, the evaluation result was problematic (C).Herein, the liquid-phase temperature T_(L) refers to a temperature atwhich generation of devitrified substances has been observed on thesurface of or in the interior of a glass after being kept melting at1550° C. for two hours followed by being kept in a temperature gradientfurnace for ten hours with a temperature gradient and rapidly cooled.Further, T_(log η=2.5) refers to a temperature at which equation logη=2.5 was held when the viscosity of a molten glass is determined usinga stirring viscometer. The evaluation results each are collectivelyshown in Table 1-Table 6.

The evaluation results confirmed that in Examples of the presentinvention in which predetermined fining agent made was contained suchthat the molar ratio to Al₂O₃ in the glass components fell in apredetermined range, the number of residual gas bubbles was remarkablysmaller than in Comparative Examples which are out of the scope of thepresent invention, and a glass substrate for an information recordingmedium with sufficiently removed gas bubbles was obtained. Further, whenthe total content of fining agents was at most 1% by mass with respectto the total glass components, the devitrification properties wereconfirmed to be excellent.

The invention claimed is:
 1. A glass substrate for an informationrecording medium made of aluminosilicate glass containing 60% to 75% bymass of SiO₂, 5% to 18% by mass of Al₂O₃, 3% to 10% by mass of Li₂O, 3%to 15% by mass of Na₂O, and 0.5% to 8% by mass of ZrO₂ with respect towhole glass components, and containing neither As (arsenic) element norSb (antimony) element, comprising: a fining agent containing SO₃(sulfurous acid), wherein a molar ratio of a whole amount of the finingagent to the Al₂O₃, which is (the whole amount of the finingagent)/Al₂O₃, is within a range of 0.02 to 0.20; and wherein a totalcontent of the fining agent is in a range of 1% by mass or less withrespect to the whole glass components.
 2. The glass substrate for aninformation recording medium of claim 1, wherein the glass substrate foran information recording medium is chemically strengthened by ionexchange.
 3. The glass substrate for an information recording medium ofclaim 1, wherein the fining agent contains at least one type selectedfrom the group consisting of F (fluorine), Br (bromine), and I (iodine).4. The glass substrate for an information recording medium of claim 1,wherein a molar ratio of a whole amount of the fining agent to the Al₂O₃is selected such that clarification reaction by a change in solubilityis adequate to sufficiently remove gas bubbles within the glasssubstrate when it is in a molten state.
 5. The glass substrate for aninformation recording medium of claim 1, wherein a molar ratio of awhole amount of the fining agent to the Al₂O₃ is selected such thatclarification reaction by a valence change is adequate to sufficientlyremove gas bubbles within the glass substrate when it is in a moltenstate.
 6. An information recording medium, comprising: a glass substratefor an information recording medium made of aluminosilicate glasscontaining 60% to 75% by mass of SiO₂, 5% to 18% by mass of Al₂O₃, 3% to10% by mass of Li₂O, 3% to 15% by mass of Na₂O, and 0.5% to 8% by massof ZrO₂ with respect to whole glass components, and containing neitherAs (arsenic) element nor Sb (antimony) element, comprising: a finingagent containing SO₃ (sulfurous acid), wherein a molar ratio of a wholeamount of the fining agent to the Al₂O₃, which is (the whole amount ofthe fining agent)/Al₂O₃, is within a range of 0.02 to 0.20; and whereina total content of the fining agent is in a range of 1% by mass or lesswith respect to the whole glass components; and a recording layerprovided on the glass substrate.
 7. The information recording medium ofclaim 6, wherein the glass substrate is chemically strengthened by ionexchange.
 8. The information recording medium of claim 6, wherein thefining agent contains at least one type selected from the groupconsisting of F (fluorine), Br (bromine), and I (iodine).
 9. Theinformation recording medium of claim 6, wherein a molar ratio of awhole amount of the fining agent to the Al₂O₃ is selected such thatclarification reaction by a change in solubility is adequate tosufficiently remove gas bubbles within the glass substrate when it is ina molten state.
 10. The information recording medium of claim 6, whereina molar ratio of a whole amount of the fining agent to the Al₂O₃ isselected such that clarification reaction by a valence change isadequate to sufficiently remove gas bubbles within the glass substratewhen it is in a molten state.